The IMF's global forecast for this year was dragged down by weaker prospects across many economies. It made big cuts to its predictions for some oil-producing nations like Nigeria and Russia, which have been suffering from the slump in oil prices.

It also dialed down its projections for many big advanced economies, including the U.S. The heaviest hits affected the outlook for Japan, Britain and Italy.

"For the first time, we're seeing the direct impact on our economy of the risks of leaving the EU," British Chancellor of Exchequer George Osborne said in a statement responding to the IMF report.

The fund also trimmed its global forecast for next year to 3.5% from its previous prediction of 3.6%.

Obstfeld called on countries around the world to come up with an "immediate response," including keeping interest rates low and investing in infrastructure to spur growth.

"If national policymakers were to clearly recognize the risks they jointly face and act together to prepare for them, the positive effects on global confidence could be substantial," he said.